1. Field of the Invention
The present invention relates to a single chip microcomputer, and more particularly to a basic clock signal generating circuit for use in the single chip microcomputer.
2. Description of Related Art
A clock used for an internal operation of the single chip microcomputer is called "basic clock", which is generally generated by frequency-dividing an input clock and waveform-shaping the frequency-divided clock. In accordance with the basic clock thus generated, the internal operations of the conventional single chip microcomputer and operation of peripheral circuits are controlled.
A typical example of the basic clock signal generating circuit for use in the single chip microcomputer basically comprises an internal frequency divider receiving an external clock signal and a waveform shaping circuit receiving an output of the frequency divider and generating a first CPU clock .phi..sub.1 and a second CPU clock .phi..sub.2, which are basic clocks for a CPU internal clock, and which are supplied to the CPU and a peripheral circuit.
For example, the frequency of the external clock signal is 8 MHz, the frequency of the clock outputted from the frequency divider is 4 MHz. Therefore, the CPU clock .phi..sub.1 and the CPU clock .phi..sub.2 are also 4 MHz and are complementary to each other. The peripheral circuit receiving these complementary CPU clocks operates with a minimum clock width corresponding to that of the CPU clocks.
As seen from the above, in the conventional single chip microcomputer, all operations are controlled on the basis of the basic clock signals, which are frequency-divided by frequency dividers for various purposes. Accordingly, the peripheral circuit cannot operate at a speed higher than that of the basic clock signal.